The irradiation of metals and semiconductor materials by ion beams provides a means for effecting the doping of such materials in a controlled and rapid manner. Ion implantation, as the process is known, is accomplished by irradiating, for example, a semiconductor wafer with an ion beam of controlled intensity for such integrated exposure as needed to provide the desired dopant or impurity concentration. See U.S. Pat. Nos. 4,283,631 and 4,383,180 issued to N. Turner for descriptions of typical ion-implantation apparatus. It is well known that for insulating wafers such as silicon-on-sapphire (SOS) wafers, or wafers that are wholly or partially covered with an insulating layer, such as silicon dioxide (SiO.sub.2) or silicon nitride (SiO.sub.3 N.sub.4), the wafers may become electrostatically charged after being implanted with positively charged ions. In such a highly charged state it becomes difficult or impossible to eject the wafers from the vacuum target chamber in the conventional automatic cassette-to-cassette silicon wafer handling system because the wafers stick to the handling apparatus.
One way to avoid the sticky effect of the wafers is to utilize a conventional electron flood system in the target chamber to neutralize the ion beam while the ions are being implanted. A typical heavy dose for semiconductor wafers is the range of 10.sup.15 to 10.sup.16 ions/cm.sup.2. However, this procedure is usually not practical or desirable for certain surface portions of ion-implanted devices that require relatively light or small dosages on the order of 10.sup.10 to 10.sup.14 ions/cm.sup.2. Such light dosages on ion-implanted devices are used, for example, for gate threshold adjustment implants in the fabrication of metal-oxide-semiconductor (MOS) circuits which require a high degree of dose accuracy and implant uniformity. Light doses are used frequently in semi-conductor processing.
The use of electron flood systems to neutralize charged wafers and other devices that have been subjected to ion-implantation or etching is known in the art. See, for example, U.S. Pat. No. 3,908,183 issued to R. M. Ennis, Jr., on Mar. 14, 1973 which describes an ion-implantation apparatus providing post-etch flooding of the device with primary electrons for cleaning and annealing the surface of debris and the like caused by an etching process.
There is a need in the art to provide a means to eliminate the electrostatic charges on silicon wafers after implantation to minimize, if not eliminate, the stickiness effect caused by ion-implantation.